Chasidut על גיטין 127:17

‎Genesis (24,12) We need to examine why Betuel and family at first ‎agreed to Rivkah’s becoming Yitzchok’s bride, (verse 50) but ‎apparently changed their minds subsequently. (Verse 55). We ‎must remember that both Lavan and Betuel, even when ‎appearing to agree, had evil intentions. (see Rashi, ‎according to whom they planned to assassinate Eliezer) Their ‎principal motivation was to prevent Yitzchok from having ‎children that would grow to maturity and survive. (Gittin 64) ‎The Talmud there discusses the subject of a betrothal by means of ‎an emissary, in the absence of a face to face meeting between ‎bride and groom. According to the halachah, as long as the ‎emissary is still on the way, i.e. has not returned from his mission, ‎the sender (Yitzchok) is not allowed to marry any other woman. ‎The reason for this is that he might, unwittingly marry someone ‎forbidden to him for reasons of incest. (In the event that his ‎emissary had already carried out his mission) For this reason, ‎Rivkah’s family first expressed their willingness, so that Yitzchok ‎was “married,” and then by killing him hoped to prevent him ‎from returning to his sender and announcing that he had ‎completed his mission.‎